A) Field of the Invention
This invention relates to a method for forming a plurality of silicon oxide films having different thicknesses on a silicon nitride film or on a silicon oxynitride film and to a method of manufacturing capacitors and a semiconductor device using that method.
B) Description of the Related Art
It is known that leaking out of boron atoms via a silicon oxynitride film is restrained by doping nitride ions to the silicon oxynitride film formed by covering one primary surface of a silicon substrate (for example, refer to Japanese Laid-Open Patent NO. Hei06-151829 which is hereinafter referred as the patent document 1).
According to the above conventional technique, density of nitrogen in a silicon oxynitride film can be increased by doping nitrogen ions; therefore, it can be avoided that boron is doped in a surface of a silicon substrate from a boron-doped poly-silicon layer via the silicon oxynitride film. However, the patent document 1 does not teach a technique for forming a silicon oxide film by performing a thermal oxidation process to a silicon oxynitride film or to a silicon nitride film.
The inventors of the present invention have tested to form a plurality of silicon oxide films having different thicknesses on a silicon nitride film by using a method for forming a silicon oxide film by performing a thermal oxidation process to a silicon nitride film. FIG. 22 to FIG. 26 show the method for forming silicon oxide films on a silicon nitride film according to the research of the inventors.
At a step shown in FIG. 22, after forming a silicon oxide film (a pad oxide film for stress relaxation) 2 on one primary surface of a silicon substrate 1 by a thermal oxidation process, a silicon nitride film 3 is formed on the silicon oxide film 2 by a chemical vapor deposition (CVD) method. Thereafter, at a step shown in FIG. 23, a silicon oxide film 4 is formed on the silicon nitride film 3 by performing a thermal oxidation process to the silicon nitride film 3.
At a step shown in FIG. 24, a resist layer 5 is formed on a part 4a of the silicon oxide film 4 by a photolithography process. Then, the silicon oxide film 4 is selectively removed by a wet-etching process with the resist layer 5 as a mask to remain the part 4a of the silicon oxide film 4 in a patter corresponding to the resist layer 5. Thereafter, at a step shown in FIG. 25, the resist layer 5 is removed by an ashing process, etc.
At a step shown in FIG. 26, the thermal oxidation process is performed to the silicon nitride film 3 again. As a result, a thick silicon oxide film 4a and a thin silicon oxide film 4b are formed on the silicon nitride film 3. Further, it is possible to make a thickness of the silicon oxide film 4b closer to a thickness of the silicon oxide film 4a at the step shown in FIG. 26 by etching the silicon oxide film 4 to remain a portion with a predetermined thickness excluding the part 4a at the step shown in FIG. 24.
According to the method for forming a silicon oxide film explained with reference to FIG. 22 to FIG. 26, it is possible to form a plurality of silicon oxide films having different thicknesses on a silicon nitride film. However, it is difficult to control the thickness of the silicon oxide film 4b precisely because a wet-etching process is not suited for controlling a film thickness, and it takes a longer process time because a plurality of thermal oxidation processes are necessary.